1. Field of the Invention
Embodiments of the present invention relate to the field of making reliable semiconductor devices, and in particular the prevention of the electromigration of copper lines.
2. Discussion of Related Art
Advances in semiconductor manufacturing technology have led to the development of integrated circuits having multiple levels of interconnect. In such an integrated circuit, patterned conductive material on one interconnect level is electrically insulated from patterned conductive material on another interconnect level by films of material such as, for example, silicon dioxide. These conductive materials are typically a metal or metal alloy. Connections between the conductive material at the various interconnect levels are made by forming openings in the insulating layers and providing an electrically conductive structure such that the patterned conductive material from different interconnect levels are brought into electrical contact with each other. These electrically conductive structures are often referred to as contacts or vias.
Other advances in semiconductor manufacturing technology have lead to the integration of millions of transistors, each capable of switching at high speed. A consequence of incorporating so many fast switching transistors into an integrated circuit is an increase in power consumption during operation. One technique for increasing speed while reducing power consumption is to replace the traditional aluminum and aluminum alloy interconnects found on integrated circuits with a metal such as copper, which offers lower electrical resistance. Those skilled in the electrical arts will appreciate that by reducing resistance, electrical signals may propagate more quickly through the interconnect pathways on an integrated circuit. Furthermore, because the resistance of copper is significantly less than that of aluminum, the cross-sectional area of a copper interconnect line, as compared to an aluminum interconnect line, may be made smaller without incurring increased signal propagation delays based on the resistance of the interconnect.
As device dimensions shrink, so does conductor width—leading to higher resistance and current density. Increasing current density leads to the phenomenon of electromigration. Electromigration is generally the movement of atoms in a metal interconnect in the direction of current flow. Most metal atoms that move during electromigration are displaced at the top of an interconnect line where there is no barrier layer to prevent their displacement. This is called surface diffusion. Surface diffusion can cause vacancies, which lead to voids and hillocks, and ultimately to electromigration failure of the device.
Others have tried to solve this problem by alloying the copper lines with another metal. One method includes the doping of the entire metal interconnect line with metallic dopants in order to prevent movement of the atoms of the metal interconnect line in the direction of the current flow. The dopants will either physically inhibit the movement of copper atoms or enlarge the copper grain size such that the diffusion path of the copper atoms is eliminated. However, blanket doping of the metal interconnect layer results in an increased resistivity of the interconnect layer, which degrades performance of the semiconductor device. In response to this increased resistivity the portion of the copper line that is doped has been decreased to only the outer edges or the top of the line to prevent surface diffusion. Shunt layers have also been used to prevent electromigration. Shunt layers are thin electrically conductive layers formed around the copper lines. Shunt layers prevent electromigration by physically inhibiting the movement of copper atoms. Additionally, shunt layers are several hundred angstroms thick and result in increased line to line leakage due to non-selective deposition. But, due to the further scaling down of devices and the narrowing of copper interconnect lines, the resistance caused by the doping of the outer layers of the lines and by the shunt layers has become significant.
Embodiments of the invention provide processes and devices that more effectively reduce electromigration, in particular surface diffusion, without significantly increasing conductor resistance. These embodiments are valuable in minimizing the electromigration of scaled down copper lines.